The present invention relates to a retainer for rolling bearings made from synthetic resin, and to a manufacturing method therefor.
For conventional synthetic-resin Trade retainers for use as retainers for rolling bearings, their raw materials have been given by those in which various synthetic resins are combined with various reinforcing materials and filler materials according to their required characteristics. Meanwhile, synthetic-resin made retainers in common use are those which are formed by fabrication with molding machines or those which are fabricated into a retainer configuration by machining work.
For example, retainers of angular contact ball bearings to be used for spindles of machine tools or the like include outer-ring guided ones. This type of retainers are required to have seizure resistance performance. To meet this characteristic requirement, we have made performance evaluations with retainers made from resin materials to which various types of filler materials (graphite, molybdenum disulfide, PTFE (polytetrafluoroethylene)) were added. As a result of this, the following problems (1) and (2) were found out:
(1) In injection molded articles, it was found that their surface was covered with a skin layer, material and resulting in earlier seizures. The terms skin layer, represent an outermost surface layer of a model article, and a surface layer made of only resin material having no filler material is herein referred to as a skin layer.
(2) In retainers whose outer configuration was made by machining work, it was found that since no skin layer was formed on the surface, allowing the filler material to be exposed at the surface and so allowing specified lubricating performance to be fulfilled, whereas cracks would be more likely to occur from the filler material as a starting point (lack of strength).
Therefore, an object of the present invention is to provides a retainer for rolling bearing, as well as a manufacturing method therefor, which allows both lubricating performance and strength to be ensure at the same time.
In order to achieve the above object, there is provided a retainer for rolling bearings, which is made from a synthetic resin base material in which a lubricant and a reinforcing material are mixed, wherein
a skin layer is provided at least part of the retainer other than inner circumferential surfaces of pockets.
In this rolling-bearing retainer, the strength is ensured by the skin layer formed over a region other than the inner circumferential surfaces of the pockets. The skin layer is not provided at the inner circumferential surfaces of the pockets serving as sliding surfaces. Therefore, the lubricant for lubricity improvement can be exposed, so that lubricating performance can be ensured. Therefore, both lubricity and strength can be satisfied at the same time.
In one embodiment of the present invention, the skin layer is absent on a guide surface.
According to the above embodiment, since no skin layer is provided at the guide surface serving as a sliding surface, the lubricant can be exposed at this guide surface, allowing the lubricity to be ensured and so making it possible to prevent seizures. Also, since the guide surface, of which strict dimensional precision is demanded, is formed by machining after injection molding, the injection molding can be performed in simple structure, which allows a cost reduction of the metal mold.
Also, there is provided a method for manufacturing a retainer for rolling bearings, comprising the steps of:
forming a ring by injection molding from a synthetic resin material in which a lubricant and a material are mixed; and
boring in the ring, circumferentialy, a plurality of holes serving as pockets for accommodating rolling elements therein by machining work.
According to the above manufacturing method of a retainer for rolling bearings, the ring is formed by injection molding, and subsequently holes serving as pockets are formed in the ring by machining work. Therefore, the skin layer is absent at the inner circumferential surfaces of the pockets, and remains over the region other than the inner circumferential surfaces of the pockets. As a result of this, the lubricity of the inner circumferential surfaces of the pockets can be ensured, while the mechanical strength as a whole can be ensured with the skin layer formed over the region other than the inner circumferential surfaces of the pockets.
Also, since the inner circumferential surfaces of the pockets, of which strict dimensional precision is demanded, is formed by machining after injection molding, the metal mold used in an injection molding can be simplified in structure, which allows a cost reduction of the metal mold.